The new method of the present invention not only achieves the aim of volume reduction, but also provides, for example, such benefits as the elimination of sulphur or chloride content from exhaust gases, and similarly any radioactive content, in an effective and straight forward manner. The invention is therefore especially useful for the processing of ionic exchange media from nuclear facilities, which media display a certain degree of radioactivity and therefore would otherwise require conventional measures in relation to ultimate waste disposal and deposition. The nuclear industry annually produces a significant amount of waste which is classified as radioactively contaminated ion exchange media. Such waste is managed in various fashions prior to ultimate disposal in bedrock chambers or shallow land burial. This management is technically complex and as a rule leads to increased volumes which influences storage costs. A process resulting in diminished volume at reasonable cost is therefore highly commercially desirable.
Ion exchange medium is an organic material. The base is usually a styrene polymer with grafted sulfonic acid and amine groups. The material is therefore burnable, but air is supplied during combustion and sulphur and nitrogen oxides are formed which in turn must be separated in some manner. Additionally, during combustion the temperature becomes sufficiently high for radioactive cesium to be partially vaporized. The residual radioactivity will also accompany the resulting fly ash to some extent. This necessitates a very high performance filter system. Accordingly, both technical and economic problems are typically associated with standard combustion techniques.
An alternative to straight combustion is pyrolysis. However, previously known pyrolysis methods in this technical field are deficient in several aspects and in particular no one has earlier succeeded in devising a pyrolysis process which provides a comprehensive solution to the problem of sulphur and nitrogen-containing radioactive waste, and to do so under acceptable economic stipulations. The following can be mentioned as examples of the known technology in this respect:
U.S. Pat. No. 5,424,042 to J. Bradley Mason et al. describes a system for vitrification of nuclear waste (incorporating of a portion of the nuclear waste into a stable glass matrix) including several subsystems: a feed conditioning system for conditioning "dry waste," "wet waste" or ion exchange resins and "liquid waste"; a feed preparation system for blending the waste types; a feed melter chamber with an upper zone and a lower zone for oxidizing the waste into ash and off-gas; a glass handling system for packing and storing the glass product; and an off-gas cleaning and control system.
U.S. Pat. No. 5,470,544 describes a system for the detoxification of hazardous waste utilizing a moving bed evaporator and a steam-reforming detoxification reactor. U.S. Pat. No. 5,427,738 also describes a system for the detoxification of solid waste which first mechanically particularized the waste in a spinning knife cutter, size reduction grinder or like device, and then subjects the particularized waste to a gas flow of a hot gas in the range of 250-750.degree. C. The particularized waste is agitated to enhance exposure to the hot gaseous flow.
SE-B 8405113-5 which describes single stage pyrolysis in a fluidised bed followed by conversion of tars in the resulting gas to non-condensable gas using limestone as catalyst.
U.S. Pat. Nos. 4,628,837, 4,636,335, and 4,654,172 describe pyrolysis of ion exchange resins where the pyrolysis is carried out in two stages. Both of these stages, however, are directed towards pyrolysis of the ion exchange media itself, i.e. the solid product. Speaking generally, both stages moreover are carried out at relatively low temperatures. Furthermore, none of these specifications recites any comprehensive solution to the problem of solid organic sulphur or chloride containing waste such as is the case with the method of the present invention.